Re: [PATCH] mm: move mask update out of the atomic context

[David Hildenbrand]

On 24.06.25 14:00, Alexander Gordeev wrote:
> On Tue, Jun 24, 2025 at 11:40:05AM +0200, David Hildenbrand wrote:
> > On 24.06.25 11:37, Alexander Gordeev wrote:
> > > On Mon, Jun 23, 2025 at 09:45:34PM +0200, David Hildenbrand wrote:
> > > ...
> > > > Let's ask the real questions: who checks PGTBL_PTE_MODIFIED?
> > > >
> > > > I see
> > > >
> > > > if (mask & ARCH_PAGE_TABLE_SYNC_MASK)
> > > > 	arch_sync_kernel_mappings(start, start + size);
> > > >
> > > > And then
> > > >
> > > > arch/arm/include/asm/page.h:#define ARCH_PAGE_TABLE_SYNC_MASK   PGTBL_PMD_MODIFIED
> > > > arch/x86/include/asm/pgtable-2level_types.h:#define ARCH_PAGE_TABLE_SYNC_MASK   PGTBL_PMD_MODIFIED
> > > > arch/x86/include/asm/pgtable-3level_types.h:#define ARCH_PAGE_TABLE_SYNC_MASK   PGTBL_PMD_MODIFIED
> > > >
> > > >
> > > > Which makes me wonder why we need PGTBL_PTE_MODIFIED at all? Is there some other check I am missing?
> > > >
> > > > (same question regarding everything excepy PGTBL_PMD_MODIFIED, because that actually seems to be used)
> > >
> > > AFAICT it was thought as architecture-specific:
> > >
> > > /*
> > >    * Architectures can set this mask to a combination of PGTBL_P?D_MODIFIED values
> > >    * and let generic vmalloc and ioremap code know when arch_sync_kernel_mappings()
> > >    * needs to be called.
> > >    */
> > > #ifndef ARCH_PAGE_TABLE_SYNC_MASK
> > > #define ARCH_PAGE_TABLE_SYNC_MASK 0
> > > #endif
> > >
> > > Not sure if that needs to be addressed at all.
> >
> > Okay, if there are no users of PGTBL_PTE_MODIFIED we could just ... remove
> > it. Dead code.
>
> As you noticed, PGTBL_PMD_MODIFIED bit is used only. Thus, all other
> bits would have to be removed as well, not just PGTBL_PTE_MODIFIED?

Likely, yes.

> That is more or less revert of at least the below commits and rewriting
> it in a PMD-focused manner:
>
> 2ba3e6947aed ("mm/vmalloc: track which page-table levels were modified")
> d8626138009b ("mm: add functions to track page directory modifications")
>
> That would be a completely different effort, which I am not aming at ;)

Well, I don't think we should be taking this micro-optimization patch 
here TBH. The real optimization is getting rid of dead code, not 
optimizing dead code.

--
Cheers,

David / dhildenb